As one of variable displacement hydraulic motors, there is a bent-axis type axial-piston motor such as one disclosed, for example in U.S. Pat. No. 3,961,563.
In FIG. 1, the reference character M denotes such bent-axis type axial-piston motor provided with a motor housing which is constructed of a main casing 1, a motor casing 2 and an end cover 3, as shown in FIG. 1. In the motor housing is obliquely mounted a cylinder block 6 so that an axis of the cylinder block 6 forms a tilt angle with an axis of a drive shaft 7, which tilt angle is variable within a range of from 7.5.degree. to 30.degree. while controlled by moving a convex portion of a valve plate 8 relative to an inner concave portion 9 of the end cover 3 in a swinging manner.
As the tilt angle is controlled, the stroke of each of a plurality of pistons 4 is so controlled as to make it possible to change a rotational speed of the drive shaft 7 in a condition in which an output horsepower of the drive shaft 7 is kept constant, provided that an amount of hydraulic oil supplied to the hydraulic motor M is kept constant.
On the other hand, in a power shovel, as shown in FIG. 1, a sleeve-like housing 12 is fixedly mounted in a bore 11 of a truck frame 10. A hub 14 fixed to a sprocket 13 is rotatably mounted on an outer peripheral portion of the housing 12 in which the hydraulic drive motor M is housed. The drive shaft 7 of the drive motor M is connected to the hub 14 through a reduction-gear mechanism 15 so that the sprocket 13 is driven by the hydraulic drive motor M.
As shown in FIG. 1, in the hydraulic drive motor M which is one of the conventional variable displacement hydraulic motors, since a slider element 3a for moving the valve plate 8 in a swinging manner is engaged with a central portion of the valve plate 8, it is impossible for the slider element 3a to smoothly swing the valve plate 8 in a direction perpendicular to an axis of the drive shaft 7. Consequently, as is clear from FIG. 1, the valve plate 8 is moved in a swinging manner by the slider element 3a in a direction slightly obliquing from the former direction perpendicular to the axis of the drive shaft 7.
The above arrangement of the slider element 3a PG,4 requires the end cover 3 to be mounted in a tilting condition, which increases the entire length of the end cover 3 and causes the motor casing 2 to be mounted in a tilting condition too to make its machining operation difficult in manufacturing.
On the other hand, in case that the abovementioned variable displacement hydraulic motor is employed as the hydraulic drive motor M, since the overall length of the drive motor M is large, a part of the drive motor M projects outward from the truck frame 10 to often interfere with obstacles such as rock and the like. In addition, in this case, since the end cover 3 is obliquely mounted, the end cover 3 tends to interfere with the truck frame 10, which makes it difficult to mount the hydraulic drive motor M in the truck frame 10, and, therefore requires a complicated mounting construction for mounting the drive motor M in the truck frame 10.